The present invention relates to an image reproducing apparatus which reproduces an image by means of electrostatic photography, and more particularly relates to an image reproducing apparatus having a means for preventing the copying operation to illegally copy a secret document of an enterprise.
Recently, color image copiers of high-performance have come into wide use. Therefore, it is anticipated that paper money and securities are frequently counterfeited using the color image copier. In order to prevent the counterfeit of paper money and securities, the following technique has been disclosed in Japanese Patent Publication Open to Public Inspection No. 111977/1980:
When a color document is digitally read out so as to be copied, a specific pattern on the document is recognized using a signal that has been read out. In the case where the recognized pattern is of paper money or securities, the copying operation is stopped.
However, in some cases, the aforementioned recognition means for recognizing the specific pattern makes a mistake.
In order to prevent the occurrence of the mistake described above, Japanese Patent Publication Open to Public Inspection No. 300285/1989 discloses the following color copier:
The color copier includes a document scanning means by which a color document is subjected to color separation and the color-separated image is digitally read out. A color separation signal outputted form the document scanning means is converted into a density signal, and in accordance with the density signal, a color image is copied. The color copier comprises: a means for recognizing a specific pattern contained in the document, using the color separation signal; and a control means for repeatedly conducting the recognizing operation of the recognizing means each time the scanning operation is conducted, wherein the control means stops the copying operation when the recognizing means recognizes the specific pattern on the document. According to the above color copier, a plurality of pattern recognizing operations are repeatedly conducted on the same document to be copied, so that the accuracy of recognition can be improved.
According to Japanese Patent Publication Open to Public Inspection No. 316783/1989, the following copier is disclosed:
The copier comprises: a reading means for reading image information from a document; an image forming means for forming an image on a recording medium in accordance with the image information that has been read by the reading means; and a judging means for judging that the read document is prohibited from being copied, wherein the read image information is subjected to converting processing in accordance with the result of the judgment.
According to the disclosure of technique explained above, using the signals of R, G and B inputted from the reading means, documents are discriminated whether or not they agree with the characteristics of paper money or securities that have been previously set. In accordance with the result of the discrimination, it is finally discriminated whether or not they are paper money or securities. In the case where the result of the discrimination is YES, a conversion parameter is set in a specific processing circuit so that the document can not be faithfully copied. In this case, the discrimination is conducted as follows:
With respect to the inputted signals of R, G and B, a color spectrum of the document is compared with the data previously registered in a ROM, or alternatively an image pattern of a portion of the document or the entire document is compared with the pattern data previously registered in the ROM. Alternatively, the aforementioned two processes may be combined.
The characteristics of a document are extracted by a characteristic extraction circuit, and in accordance with the result of the characteristic discrimination, parameter conversion processing is carried out. With respect to the parameter conversion processing, the following methods are disclosed:
(1) Color conversion conducted according to a parameter of masking and UCR processing
(2) Conversion of magnification, italic characters and mirror images
(3) Edge emphasis, fine line processing, negative and positive conversion, and character pattern addition.
In order to prevent the counterfeit of paper money or securities when a document is copied in a printer, the following methods are disclosed: Video signals are sequentially monitored, and when it is judged that a document is paper money or securities, the images are subjected to the processing of thinning out, fine line, bold line, and screen pattern.
Further, according to Japanese Patent Publication Open to Public Inspection No. 55379/1990, the following method is disclosed:
For example, the pattern of a seal put on paper money or written contract, which appropriately expresses the characteristics of a specific document, is previously stored in the form of template information. When the template information is compared with an image outputted after processing, it is Judged whether or not it is a case of counterfeit. Specifically, the signals of R, G and B sent from the image reading unit pass through a comparator and AND gate. In the case of R.sub.L &lt;R.ltoreq.R.sub.H, G.sub.L &lt;G.ltoreq.G.sub.H, and B.sub.L &lt;B.ltoreq.B.sub.H, it is judged that the color is a specific one. For example, it is judged that the image is a red seal of paper money.
The digital image signals R, B and G are inputted into the characteristic extraction circuit. The characteristic extraction circuit targets a specific pattern and color distribution, and conducts characteristic extraction processing. According to the result of the processing, a portion or an entire drive unit necessary for image formation is stopped. This technique is disclosed in Japanese Patent Publication Open to Public Inspection No. 210481/1990.
According to another known system, a document is provided with a mark by which the document can be easily recognized as an original sheet to be copied, or alternatively the document is provided with a second mark. Only when the copier detects the aforementioned mark or the second mark on the original sheet to be copied, copying operation is conducted. When the mark is not detected, the original sheet to be copied is not conveyed, or even when the original sheet to be copied is conveyed, image formation is not conducted, and the original sheet to be copied is discharged from the copier as it is, and a display means displays that the original sheet to be copied has been discharged. In this way, the counterfeit of paper money and securities can be prevented.
However, according to the above applications for preventing counterfeit, it is essentially necessary to provide a technique by which a specific pattern can be recognized. Therefore, the construction is complicated and the cost is increased, which is the first problem. Further, the conventional technique is mainly applied to digitally operated copiers.
In the case of a copier including a document scanning means by which a color document is subjected to color-separation and read out digitally, also including a means for recognizing a specific pattern on the document using a color-separation signal outputted from the document scanning means, the recognizing operation is affected by color ghost, so that a problem of malfunction is caused.
The phenomenon and cause of color ghost will be briefly described here. In the color copier, a document image is optically subjected to color-separation, so that it is separated to red and blue. Then, the red and blue images are respectively read out. In accordance with a signal sent from an individual image sensor (referred to as CCD hereinafter), a color of one pixel is determined. Therefore, when the position of CCD of red and that of blue deviate, a false signal of red or blue is generated although a line is black, which is referred to as color ghost. The causes of color ghost will be described as follows:
(1) Color ghost is caused due to pixel deviation between the CCDs caused when the accuracy of the mounted CCD is inappropriate, or when the CCD is deformed, or when the CCD deteriorates with age.
(2) Color ghost is caused when the magnification of red and that of blue are different, or when MTF of red and that of blue are different.
(3) Color ghost is caused when the output of red and that of blue are different due to chromatic aberration of a lens.
According to the recognition technique in which color-separation signals sent from the document scanning means are used, the characteristics of a specific document are made into a pattern so as to be compared. Therefore, a ratio of discrimination for discriminating the specific document is low, which is the second problem. In this case, MTF (Modulation Transfer Function) will be explained as follows.
A rate of deterioration of an image can be evaluated when the image is compared with the original document. However, its evaluation is not quantitative but qualitative. In order to quantitatively evaluate the rate of deterioration of an image, MTF used for evaluating a signal transfer system is adopted.
The modulation transfer function is expressed as follows. EQU MTF(%)=(V.sub.max -V.sub.min)/(V.sub.max +V.sub.min).times.100
In the above expression, V.sub.max is the maximum density level of a substantial deteriorated waveform in the case where the waveform is read out by the CCD, and V.sub.min is the minimum density level of a substantial deteriorated waveform in the case where the waveform is read out by the CCD. In this case, signal deterioration is explained in the case where the image is read out by the CCD. In general, the MTF deteriorates while the image is recorded and reproduced. Deterioration factors of the MTF are an optical system, optical traveling system, processing circuit, and recording system. The following directly affect the deterioration of resolution: the MTF of the lens in the optical scanning system; the accuracy of a prism surface; the accuracy of mounting the CCD; the vibration of an optical mirror in the optical traveling system; and scanning speed.
Characteristics of the MTF are described as follows.
(1) Compared with the primary scanning direction, the MTF remarkably deteriorates in the subsidiary scanning direction.
(2) In order to improve the reproduction property of the finest portion of an image, the MTF value must be not less than 30%.
(3) Intensity of MTF correction must be determined in accordance with the reproduction property of fine lines and the photographic reproducing property.
According to the above recognition technique in which the color-separation signal sent from the document scanning means is used, a specific mark for specifying a document is blurred in some cases. In this case, it becomes difficult to extract the specific mark.
In the case where there are a large number of specific documents, it takes a good deal of time for the step of specifying the document and the step of judging the coincidence the specific document. Accordingly, the performance of the entire image forming apparatus is lowered, which is the third problem.
According to the image reproducing apparatus of the prior art and also according to the image reproducing apparatus of the present invention, it is possible to prevent a specific document from being copied. However, it is necessary for a specific person to copy the specific document for the purpose of providing information. In other words, information must be provided to the specific person. On the other hand, information must be kept secret from other persons. Specifically, the specific document is permitted to be copied by the first generation, and prohibited from being copied by the second and after generations.